As integrated circuit features are scaled down and density increases, material properties such as resistivity, which influences observed resistance, exhibit relatively more pronounced effects. For example, as feature size drops, interconnect delay may exceed gate delay and form a relatively large portion of total device delay. Interconnect delay is understood to be caused, at least in part, by resistive-capacitance delay. Resistive-capacitance delay, or RC delay, is understood as the delay of signal propagation as a function of resistance, which is in part dependent on the resistivity of the metal wire composition, and as a function of insulator capacitance, which is in part dependent on the permittivity of the interlayer dielectric. Prior solutions to reduce RC delay included wire geometry optimization.
Furthermore, reliability of integrated circuits is affected by a number of stresses that increase as feature size drops and density increases. These stresses include electrical, thermal, mechanical and environmental stress. Electromigration is an example of phenomena that reduces semiconductor reliability, leads to interconnect failure, and becomes relatively more prominent as feature size is decreased and power density increases. Electromigration is understood as the transport of material due to movement of ions in a conductor. Electromigration may result in the formation of hillocks or voids in the interconnects and eventually lead to failure.
To reduce electromigration, and other stress induced failures, refractory metals have been used in interconnect fabrication. However, refractory metals exhibit increased resistivity and, therefore, increased resistance, increasing resistive-capacitance delay. To further reduce electromigration, and other stress induced failures, diffusion barriers have been deposited on the side and bottom walls of openings in interlayer dielectrics containing the interconnects. Diffusion barriers are understood to occupy a small fraction (typically 20% or less) of the cross-sectional area of the interconnect wire. The portion of the interconnect exposed at the surface of a given dielectric layer is coated with an insulator such as silicon nitride. However, such an arrangement may adversely affect wire properties such as capacitance.
Therefore, as feature sizes continue to decrease, room remains for the improvement in the design of interconnects with, in some instances, an emphasis on interconnect delay and resistance to various stresses, such as those resulting in electromigration and thermomechanical failures.